Equivalence ratio-based system for controlling transient fueling in an internal combustion engine

ABSTRACT

An equivalence ratio-based system for controlling transient engine fueling includes an engine controller responsive to a number of engine operating conditions to estimate a mass of oxygen trapped within a number of cylinders of an internal combustion engine. The engine controller is further operable to map current values of engine speed and commanded fueling to one of a number of predetermined maximum fuel-to-oxygen, or equivalence, ratio values (Φ MAX ). The engine controller is then operable to determine an oxygen/fuel control (OFC) limited fueling command (F OFCL ) as a function of the estimated oxygen mass value and the maximum equivalence ratio, and to limit engine fueling based on the OFC limited fueling command F OFCL . In one embodiment, the engine controller is operable to fuel the engine according to a minimum of the OFC limited fueling command F OFCL  and a default fueling command F DEF , although other fuel limiting strategies are contemplated.

FIELD OF THE INVENTION

[0001] The present invention relates generally to fuel managementsystems for internal combustion engines, and more specifically to suchsystems for controlling transient particulate emissions by controlling atransient fuel-to-oxygen, or equivalence, ratio.

BACKGROUND OF THE INVENTION

[0002] When combustion occurs in an environment with excess oxygen, peakcombustion temperatures increase which leads to the formation ofunwanted emissions, such as oxides of nitrogen (NO_(X)). Particulateemissions are likewise generally undesirable, and the amount oftransient particulate emissions produced by an engine is largely afunction of the transient peak overall fuel-to-oxygen, or equivalence,ratio (Φ). Unfortunately, both problems are aggravated through the useof turbocharger machinery operable to increase the mass of fresh airflow, and hence increase the concentrations of oxygen and nitrogenpresent in the combustion chamber when temperatures are high during orafter the combustion event.

[0003] One known technique for reducing unwanted NO_(X) emissionsinvolves introducing chemically inert gases into the fresh air flowstream for subsequent combustion. By thusly reducing the oxygenconcentration of the resulting charge to be combusted, the fuel burnsslower and peak combustion temperatures are accordingly reduced, therebylowering the production of NO_(X). In an internal combustion engineenvironment, such chemically inert gases are readily abundant in theform of exhaust gases, and one known method for achieving the foregoingresult is through the use of a so-called Exhaust Gas Recirculation (EGR)system operable to controllably introduce (i.e., recirculate) exhaustgas from the exhaust manifold into the fresh air stream flowing to theintake manifold.

[0004] Constraining particulate emissions, on the other hand, requirescarefully controlling the equivalence ratio (Φ), particularly duringtransient operating conditions. However, no systems are currently knownfor accurately estimating in-cylinder oxygen content in dynamic fuel/O₂environments that are generally characteristic of EGR-based systems.Accordingly, no accurate equivalence ratio-based fuel control systemsare known to exist. What is therefore needed is a system for accuratelydetermining in-cylinder oxygen content, and for controlling thefuel-to-oxygen, or equivalence, ratio Φ based on this information aswell as on other current operating conditions to thereby minimizetransient particulate emissions while optimizing transient torquecapability in a dynamic fuel/O₂ environment that is characteristic ofEGR-based systems.

SUMMARY OF THE INVENTION

[0005] The foregoing shortcomings of the prior art are addressed by thepresent invention. In accordance with one aspect of the presentinvention, an equivalence ratio-based system for controlling transientfueling in an internal combustion engine comprises an engine speedsensor producing an engine speed signal indicative of rotational speedof an internal combustion engine, means for determining a quantity ofoxygen trapped within a number of cylinders of the engine and producingan oxygen estimate corresponding thereto, and a control circuitproducing a fueling command for fueling the engine and determining amaximum equivalence ratio value based on the fueling command and theengine speed signal, the control circuit limiting the fueling commandbased on the maximum equivalence ratio and the oxygen estimate.

[0006] In accordance with another aspect of the present invention, anequivalence ratio-based system for controlling transient fueling in aninternal combustion engine comprises an engine speed sensor producing anengine speed signal indicative of rotational speed of an internalcombustion engine, means for determining a residual mass valuecorresponding to a mass of residual gases trapped within a number ofcylinders of the engine, means for producing a fueling command forfueling the engine, means responsive to the residual mass value, theengine speed signal and the fueling command for determining a quantityof oxygen trapped within the number of cylinders of the engine andproducing an oxygen value corresponding thereto, and a control circuitlimiting the fueling command based on the engine speed signal, thefueling command and the oxygen value.

[0007] In accordance with yet another aspect of the present invention,an equivalence ratio-based method for controlling transient fueling inan internal combustion engine comprises the steps of sensing rotationalspeed of an internal combustion engine and producing an engine speedsignal corresponding thereto, determining a maximum equivalence ratiovalue based on an engine fueling command and the engine speed signal,determining a quantity of oxygen trapped within a number of cylinders ofthe engine and producing an oxygen value corresponding thereto, andlimiting fuel supplied to the engine command based on the maximumequivalence ratio and the oxygen value.

[0008] In accordance with still another aspect of the present invention,an equivalence ratio-based method for controlling transient fueling inan internal combustion engine comprises the steps of sensing rotationalspeed of an internal combustion engine and producing an engine speedsignal corresponding thereto, determining a residual mass valuecorresponding to a mass of residual gases trapped within a number ofcylinders of the engine, producing a fueling command for fueling theengine, determining a quantity of oxygen trapped within the number ofcylinders of the engine based on the engine speed, the residual massvalue and the fueling command and producing an oxygen valuecorresponding thereto, and limiting the fueling command based on theengine speed signal, the fueling command and the oxygen value.

[0009] One object of the present invention is to provide a fuelingcontrol system for minimizing particulate emissions while optimizingengine output torque capabilities under transient operating conditions.

[0010] Another object of the present invention is to provide such asystem for achieving the foregoing object in a dynamic fuel-oxygenenvironment characteristic of EGR-based systems.

[0011] Still another object of the present invention is to provide afuel control system operable to achieve the foregoing objects bycontrolling a maximum fuel-to-oxygen, or equivalence, ratio (Φ) based ona computed amount of oxygen trapped within a number of cylinders of theengine as well as on other engine operating conditions.

[0012] These and other objects of the present invention will become moreapparent from the following description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a diagrammatic illustration of one preferred embodimentof an equivalence ratio-based system for controlling transient fuelingin an internal combustion engine, in accordance with the presentinvention.

[0014]FIG. 2 is a diagrammatic illustration of one preferred embodimentof a technique for determining a charge flow parameter for use by theequivalence ratio fuel limiter block of FIG. 1.

[0015]FIG. 3 is a diagrammatic illustration of one preferred embodimentof the equivalence ratio fuel limiter block of FIG. 1, in accordancewith the present invention.

[0016]FIG. 4 is a diagrammatic illustration of one preferred embodimentof the residual mass estimator block of FIG. 3, in accordance with thepresent invention.

[0017]FIG. 5 is a diagrammatic illustration of one preferred embodimentof the in-cylinder oxygen estimator block of FIG. 3, in accordance withthe present invention.

[0018]FIG. 6 is a diagrammatic illustration of one preferred embodimentof the OFC fuel limit estimator block of FIG. 3, in accordance with thepresent invention.

[0019]FIG. 7 is a plot illustrating a number of example relationshipsbetween maximum equivalence ration and engine speed, in accordance withthe present invention.

[0020]FIG. 8 is a flowchart illustrating one preferred embodiment of analgorithm for carrying out some of the concepts of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] For the purposes of promoting an understanding of the principlesof the invention, reference will now be made to a number of preferredembodiments illustrated in the drawings and specific language will beused to describe the same. It will nevertheless be understood that nolimitation of the scope of the invention is thereby intended, suchalterations and further modifications in the illustrated embodiments,and such further applications of the principles of the invention asillustrated therein being contemplated as would normally occur to oneskilled in the art to which the invention relates.

[0022] Referring now to FIG. 1, one preferred embodiment of a system 10for controlling engine exhaust temperature, in accordance with thepresent invention, is shown. System 10 includes an internal combustionengine 12 having an intake manifold 14 fluidly coupled to a compressor18 of a turbocharger 20 via intake conduit 16, wherein the compressor 18receives fresh air via fresh air conduit 22. Optionally, as shown inphantom in FIG. 1, system 10 may include an intake air cooler 24 ofknown construction disposed in line with intake conduit 16 between theturbocharger compressor 18 and the intake manifold 14. The turbochargercompressor 18 is mechanically coupled to a turbocharger turbine 28 viadrive shaft 26, wherein turbine 28 is fluidly coupled to an exhaustmanifold 30 of engine 12 via exhaust conduit 32, and is further fluidlycoupled to ambient via exhaust conduit 34. The exhaust conduit 32 isfluidly coupled to intake conduit 16 via an EGR conduit 36 having a flowrestriction structure disposed in-line with conduit 36. In theembodiment illustrated in FIG. 1, the flow restriction structure is anEGR valve 38, although the present invention contemplates providing foralternative or additional flow restriction structures along EGR conduit38. In any case, an EGR cooler 40 of known construction may optionallybe disposed between EGR valve 38 and intake conduit 16 as shown inphantom in FIG. 1.

[0023] System 10 includes an engine controller 42 that is preferablymicroprocessor-based and is generally operable to control and manage theoverall operation of engine 12. Engine controller 42 includes a numberof inputs and outputs for interfacing with various sensors and systemscoupled to engine 12, and in one embodiment, controller 42 may be aknown control unit sometimes referred to as an electronic or enginecontrol module (ECM), electronic or engine control unit (ECU) or thelike, or may alternatively be a general control circuit capable ofoperation as described hereinafter. In any case, control circuit 20includes a default fueling block 34 receiving the engine speed signal(ESP) from engine speed sensor 26 via signal path 28, as well as anumber of additional input signals 36. Block 34 is responsive to the ESPsignal on signal path 28 as well as one or more of the additionalsignals 36 to compute a default fueling command (DFC) in accordance withtechniques well-known in the art. The default fueling command DFC may bean unrestricted fueling command that is used as the final fuelingcommand FFC produced on any number, M, of signal paths 42 forcontrolling fuel system 40, wherein M may be any positive integer. As itrelates to the present invention, however, the default fuelingdetermination block 34 may alternatively or additionally include one ormore fuel limiting algorithms designed to achieve certain engineoperational goals, wherein the default fueling command DFC produced byblock 34 represents an unrestricted fueling command that has beenlimited by one or more such fuel limiting algorithms.

[0024] In accordance with the present invention, engine controller 42includes an equivalence ratio fuel limiter block 44 receiving a numberof input signals and values and producing an oxygen/fuel control (OFC)limited fueling command F_(OFCL) that is also shown in FIG. 1 as beingprovided to other control blocks within engine controller 42 as anoxygen/fuel control limit value OFC LIMIT. As used herein, the term“equivalence ratio” (represented by the symbol Φ) is defined as thefuel-to-oxygen ratio of charge entering the intake manifold 14 of engine12. In accordance with the present invention, block 44 is operable tocontrol ( ) by controlling the OFC limited fueling command F_(OFCL)based on information relating to current values of total oxygen trappedwithin the cylinders of engine 12 and other engine operating conditions.In general, the values of the OFC limited fueling command F_(OFCL) arechosen such that the resulting equivalence ratio Φ yields transientparticulate emissions below to a desired level while also optimizingtransient engine output torque. Engine controller 42 also includes adefault fueling block 78 producing a default fueling command F_(DEF),wherein the OFC limited fueling command F_(OFCL) and the default fuelingcommand F_(DEF) are provided as inputs to a MIN block 80. MIN block 80is operable to produce a final fueling command FFC on signal path 84that is a minimum of the OFC limited fueling command F_(OFCL) and thedefault fueling command F_(DEF). Fuel system 82 is responsive to thefinal fueling command FFC to supply fuel to engine 12 as is known in theart. In a general sense, the default fueling command F_(DEF) ispreferably determined in a known manner and represents a conventionalfueling command determined and asserted by engine controller 42 based ona number of engine operating conditions as is known in the art.

[0025] System 10 includes a number of sensors and sensing systems forproviding the engine controller 42 with information relating to theoperation of engine 12. For example, the system 10 includes adifferential pressure sensor 46 having one input fluidly connected toEGR conduit 36 adjacent to one end of EGR valve 38 via conduit 48, andan opposite input fluidly connected to EGR conduit 36 adjacent to anopposite end of EGR valve 38 via conduit 50. Differential pressure (ΔP)sensor 46 is preferably of known construction and is electricallyconnected to a ΔP input of the equivalence ratio fuel limiter block 44via signal path 52. In operation, the ΔP sensor 46 is operable toproduce a differential pressure signal on signal path 52 indicative of adifferential pressure across EGR valve 38. An intake manifold pressure(IMP) sensor 54 of known construction is disposed in fluidcommunications with the intake manifold 14 and is electrically connectedto an intake manifold pressure (IMP) input of the equivalence ratio fuellimiter block 44 via signal path 56. The IMP sensor 54 is operable toproduce a pressure signal on signal path 56 indicative of intakemanifold pressure. As will be described in greater detail hereinafter,the equivalence ratio fuel limiter block 44 is operable to determine theOFC limited fueling command F_(OFCL) as a function of, among otherengine operating conditions, the pressure of exhaust gas produced byengine 12, and in one embodiment block 44 is operable to determineexhaust pressure as a sum of the differential pressure signal producedby the ΔP sensor 46 and the intake manifold pressure signal produced bythe IMP sensor 54. Alternatively, system 10 may include a known exhaustpressure sensor (EXP) 56 in fluid communications with the exhaustmanifold 30 or exhaust conduit 32 and electrically connected to anexhaust pressure input (EXP) of block 44 via signal path 60 as shown inphantom in FIG. 1. In this embodiment, the EXP sensor 58 is operable toprovide the equivalence ratio fuel limiter block 44 with a signalindicative of exhaust pressure, and the IMP sensor 54 and ΔP sensor 46may thus be omitted from system 10. In an alternative embodiment, enginecontroller 42 may includes a known exhaust pressure estimation strategyoperable to produce an estimated exhaust pressure value, wherein theestimated exhaust pressure value is provided to the exhaust pressureinput EXP of block 44.

[0026] System 10 also includes an engine speed sensor 62 operable tosense rotational speed of the engine 12 and produce an engine speedsignal on signal path 64 indicative of engine rotational speed. In oneembodiment, sensor 62 is a Hall effect sensor operable to determineengine speed by sensing passage thereby of a number of equi-angularlyspaced teeth formed on a gear or tone wheel. Alternatively, engine speedsensor 62 may be any other known sensor operable as just describedincluding, but not limited to, a variable reluctance sensor or the like.In any case, the engine speed signal provided on signal path 64 issupplied as an engine speed (ES) input to the equivalence ratio fuellimiter block 44 of engine controller 42.

[0027] The equivalence ratio fuel limiter block 44 of engine controller42 also includes an exhaust temperature input (EXT) configured toreceive a signal or value indicative of the current temperature ofexhaust gas produced by engine 12. In one embodiment, engine controller42 preferably includes an exhaust temperature estimation strategyoperable to produce an estimated exhaust temperature value (ETE),wherein the ETE value is provided to the exhaust temperature input EXTof block 44. A preferred exhaust temperature estimation strategy for usewith the present invention is described in U.S. patent application Ser.No. ______, entitled SYSTEM FOR CONTROLLING ENGINE EXHAUST TEMPERATURE,which is assigned to the assignee of the present invention and thedisclosure of which is incorporated herein by reference, although thepresent invention contemplates using other known exhaust temperaturedetermination strategies. In an alternative embodiment of the presentinvention, system 10 may include a known exhaust temperature sensor(EXT) 66 in fluid communications with the exhaust manifold 30 or exhaustconduit 32 and electrically connected to the exhaust temperature input(EXT) of block 44 via signal path 68 as shown in phantom in FIG. 1. Inthis embodiment, the EXT sensor 66 is operable to provide theequivalence ratio fuel limiter block 44 with a signal indicative ofexhaust temperature, and the exhaust temperature estimation strategywithin engine controller 42 may thus be omitted.

[0028] The equivalence ratio fuel limiter block 44 of engine controller42 also includes an exhaust gas recirculation flow (EGRF) inputconfigured to receive a signal or value indicative of EGR mass flowsupplied by the exhaust conduit 32 to the intake conduit 16 via EGRconduit 36. In one embodiment, engine controller 42 preferably includesan EGR mass flow estimation strategy operable to produce an estimatedEGR mass flow value (EGRF), wherein the EGRF value is provided to theEGR flow input EGRF of block 44. A preferred EGR mass flow estimationstrategy for use with the present invention is described in U.S. patentapplication Ser. No. ______, entitled SYSTEM AND METHOD FOR ESTIMATINGEGR MASS FLOW AN EGR FRACTION, which is assigned to the assignee of thepresent invention and the disclosure of which is incorporated herein byreference, although the present invention contemplates using other knownEGR mass flow determination strategies. In an alternative embodiment ofthe present invention, system 10 may include a known mass flow sensor 74in fluid communications with the EGR conduit 36 preferably between theEGR valve 38 and the intake conduit 16, and electrically connected tothe EGR flow input (EGRF) of block 44 via signal path 76 as shown inphantom in FIG. 1. In this embodiment, the mass flow sensor 74 isoperable to provide the equivalence ratio fuel limiter block 44 with asignal indicative of EGR mass flow, and the EGR mass flow estimationstrategy within engine controller 42 may thus be omitted.

[0029] The equivalence ratio fuel limiter block 44 of engine controller42 also includes a charge flow (CHF) input configured to receive asignal or value indicative of the mass flow of charge supplied to theintake manifold 14 of engine 12 and a volumetric efficiency (η) inputconfigured to receive a value indicative of the volumetric efficiency ofthe intake manifold 14. In one embodiment, engine controller 42preferably includes a charge flow estimation strategy operable toproduce an estimated mass charge flow value (CHF) and an estimatedvolumetric efficiency value (η), and one embodiment of a system forestimating mass charge flow and volumetric efficiency is illustrated inFIG. 2.

[0030] Referring now to FIG. 2, one preferred embodiment of a system 100for estimating mass charge flow and volumetric efficiency is shown. Theterm “charge”, as used herein, is defined as a composition of fresh airsupplied by conduit 16 via turbocharger compressor 18 and exhaust gassupplied by exhaust conduit 32 via EGR valve 38, and mass charge flow isthus the mass flow of charge supplied to the intake manifold 14 ofengine 12. System 100 includes several components in common with system10 of FIG. 1, and like numbers are therefore used to identify likecomponents. For example, system 100 includes an internal combustionengine 12 having an intake manifold 14 fluidly coupled to a compressor18 of a turbocharger 20 (not shown) via intake conduit 16, whereinconduit 16 receives fresh air via the turbocharger compressor asdescribed with respect to FIG. 1. An exhaust manifold 30 of engine 12expels exhaust gas to ambient via exhaust conduit 32, and an EGR valve38 is preferably disposed in fluid communication with the intake andexhaust conduits 16 and 32 respectively via EGR conduit 36. A ΔP sensor46 is preferably positioned across the EGR valve 38 via conduits 48 and50, and is electrically connected to an input of a charge flowdetermination block 102 of engine controller 42 via signal path 52. Anintake manifold pressure sensor 54 is connected to another input of thecharge flow determination block 102 via signal path 56, and an enginespeed sensor 62 is electrically connected to another input of block 102via signal path 64.

[0031] An intake manifold temperature sensor (IMT) 104 is disposed influid communication with the intake manifold 14 of engine 12, and iselectrically connected to another input of the charge flow determinationblock 102 of engine controller 44 via signal path 106. IMT sensor 104 ispreferably a known sensor operable to produce a signal on signal path106 corresponding to the temperature of charge flowing into the intakemanifold 14. Optionally, as shown and described with respect to FIG. 1,system 100 may include an exhaust pressure sensor EXP 68 disposed influid communication with the exhaust manifold 30 or exhaust conduit 32,as shown in phantom in FIG. 2, wherein either sensor 68 may be connectedto the ΔP input of block 102.

[0032] In one preferred embodiment, the charge flow determination block102 of the engine controller 42 is operable to compute an estimate ofthe mass flow of charge (ECF) into intake manifold 14 by firstestimating the volumetric efficiency (η) of the charge intake system,and then computing ECF as a function of η using a conventionalspeed/density equation. Any known technique for estimating η may beused, and in one preferred embodiment of block 102 η is computedaccording to a known Taylor mach number-based volumetric efficiencyequation given as:

η=A ₁*{(Bore/D)²*(stroke*ES)^(B) /sqrt(γ*R*IMT)*[(1+EXP/IMP)+A ₂ ]}+A₃  (1),

[0033] where,

[0034] A₁, A₂, A₃ and B are all calibratable parameters preferably fitto the volumetric efficiency equation based on mapped engine data,

[0035] Bore is the intake valve bore length,

[0036] D is the intake valve diameter, stroke is the piston strokelength, wherein Bore, D and stroke are generally dependent upon enginegeometry,

[0037] γ and R are known constants (γ*R=387.414 KJ/kg/deg K),

[0038] ES is engine speed,

[0039] IMP is the intake manifold pressure,

[0040] EXP is the exhaust pressure, where EXP=IMP+ΔP, and

[0041] IMT=intake manifold temperature.

[0042] From the foregoing equation, it should be apparent that system100 may substitute an exhaust pressure sensor 68, as shown in phantom inFIG. 2, for the ΔP sensor 46, although commercially available exhaustpressure sensors that are capable of withstanding harsh environmentsassociated with the exhaust manifold 30 and/or exhaust conduit 32 arenot typically available. For purposes of the present invention, a ΔPsensor 46 is therefore preferably used.

[0043] With the volumetric efficiency value η estimated according toequation (1), the estimate charge flow value ECF is preferably computedaccording to the equation:

ECF=η*V _(DIS) *ES*IMP/(2*R*IMT)  (2),

[0044] where,

[0045] η is the estimated volumetric efficiency,

[0046] V_(DIS) is engine displacement and is generally dependent uponengine geometry,

[0047] ES is engine speed,

[0048] IMP is the intake manifold pressure,

[0049] R is a known gas constant (R=54), and

[0050] IMT is the intake manifold temperature.

[0051] Referring again to FIG. 1, those skilled in the art willrecognize that a mass air flow sensor 70 of known construction mayalternatively be disposed within the intake manifold 14, whereininformation provided by such a mass air flow sensor on signal path 72may be used to determine mass charge flow directly rather than using acharge flow virtual sensor as just described.

[0052] Referring now to FIG. 3, one preferred embodiment of theequivalence ratio fuel limiter block 44 of FIG. 1, in accordance withthe present invention, is shown. In one embodiment, block 44 preferablyincludes a summation block 150 having a first input receiving thedifferential pressure signal (ΔP) from signal path 52, a second inputreceiving the intake manifold pressure signal (IMP) from signal path 56,and an output producing an exhaust pressure value as a sum of the ΔP andIMP signals, wherein the exhaust pressure output value of block 150 isprovided to an exhaust pressure (EXP) input of a residual mass estimatorblock 152. Alternatively, as described hereinabove with respect to FIG.1, system 10 may include an exhaust pressure sensor 58 and the exhaustpressure (EXP) input of the residual mass estimator block 152 may thusbe configured to receive an exhaust pressure signal directly from sensor58 via signal path 60. In this embodiment, sensors 46 and 54 may beomitted from system 10 for purposes of the present invention. In anycase, an exhaust temperature (EXT) input of the residual mass estimatorblock 152 is configured, in one preferred embodiment, to receive anestimated exhaust temperature value (ETE) supplied by an engine exhausttemperature estimation strategy included within the engine controller 42as described hereinabove with respect to FIG. 1. Alternatively, system10 may include an engine exhaust temperature sensor 66 as describedabove, and the exhaust temperature (EXT) input of the residual massestimator block 152 may thus be configured to receive an exhausttemperature signal directly from sensor 66 via signal path 68.Regardless of the sources of the exhaust pressure and exhausttemperature signals or values, the residual mass estimator block 152 isoperable to compute a residual mass value (RM) as a function of theexhaust pressure and exhaust temperature values and to supply theresidual mass value (RM) to a residual mass input (RM) of an in-cylinderoxygen estimator block 154. In accordance with the present invention,the residual mass value (RM) corresponds to a mass of residual gasestrapped within the cylinders of engine 12, and one preferred strategyfor computing RM will be fully described hereinafter with respect toFIG. 4.

[0053] The in-cylinder oxygen estimator block 154 includes charge flow(CHF) and volumetric efficiency (η) inputs preferably configured toreceive an estimated charge flow value (ECF) and an estimated volumetricefficiency value (η) respectively supplied by a charge flow estimationstrategy included within the engine controller 42 as describedhereinabove with respect to FIG. 2. Alternatively, system 10 may includea mass flow sensor 70 as described above, and the charge flow (CHF)input of the in-cylinder oxygen estimation block 154 may thus beconfigured to receive a mass charge flow signal directly from sensor 70via signal path 72. The in-cylinder oxygen estimator block 154 furtherincludes an EGR mass flow (EGRF) input preferably configured to receivean estimated EGR mass flow value (EGRF) supplied by an EGR mass flowestimation strategy included within the engine controller 42 asdescribed hereinabove with respect to FIG. 1. Alternatively, system 10may include a mass flow sensor 74 as described above, and the EGR massflow (EFRF) input of the in-cylinder oxygen estimation block 154 maythus be configured to receive an EGR mass flow signal directly fromsensor 74 via signal path 76. The in-cylinder oxygen estimator block 154further includes an engine speed (ES) input configured to receive theengine speed signal on signal path 64 and a commanded fueling (CF) inputconfigured to receive the default fueling command F_(DEF) from thedefault fueling block 78 of FIG. 1. The in-cylinder oxygen estimatorblock 154 is operable to compute an in-cylinder oxygen value (ICO2) as afunction of the residual mass (RM), charge flow (CHF), EGR mass flow(EGRF), default fueling (F_(DEF)), volumetric efficiency (η), and enginespeed (ES) values, and to supply the in-cylinder oxygen value (ICO2) toan oxygen input (O2) of an oxygen/fuel control (OFC) fuel limitestimator block 156. In accordance with the present invention, thein-cylinder oxygen value (ICO2) corresponds to a mass of oxygen trappedwithin the cylinders of engine 12, and one preferred strategy forcomputing ICO2 will be fully described hereinafter with respect to FIG.5.

[0054] The OFC fuel limit estimator block 156 further includes commandedfuel (CF) input configured to receive the default fueling value(F_(DEF)) from block 78 (FIG. 1) and an engine speed input (ES)configured to receive the engine speed signal on signal path 64. The OFCfuel limit estimator block 156 is operable to compute the OFC limitedfueling command F_(OFCL) as a function of the in-cylinder oxygen value(ICO2), the commanded fueling value (CF) and the engine speed signal(ES), as will be described in detail hereinafter with respect to FIGS. 6and 7.

[0055] Referring now to FIG. 4, one preferred embodiment of the residualmass estimator block 152 of FIG. 3, in accordance with the presentinvention, is shown. Block 152 includes an arithmetic block 160 having amultiplication input receiving a mass density constant stored in block162. In one embodiment, the mass density constant (MDC) is a function ofengine geometry and is given by the equation:

MDC=(DIS*K1)/[(CR−1)*R*K2*NCYL]  (3),

[0056] where,

[0057] DIS is cylinder displacement (in³),

[0058] K1 is a constant (453,600 mg/lbm),

[0059] CR is the cylinder compression ratio,

[0060] R is a gas constant (54 ft-lbf/lbm-° R),

[0061] K2 is a conversion constant (12 in/ft), and

[0062] NCYL is the number of cylinders in the engine.

[0063] A second multiplication input of arithmetic block 160 receivesthe exhaust pressure value EXP provided by summation block 150 orexhaust pressure sensor 58. A division input of block 160 receives anexhaust temperature value from summation block 164, wherein block 164 isoperable to sum the exhaust temperature value ETE provided by an exhausttemperature estimation algorithm or exhaust temperature sensor 66 and atemperature conversion value stored within block 166. In a preferredembodiment of the present invention, exhaust temperature is used inunits of degrees-R, and block 166 accordingly holds a conversion value(e.g., 460) for converting degrees-C to degrees-R. The output ofarithmetic block 160 is provided as an input to a known limiter 168having a second input receiving a maximum mass value from block 170 anda third input receiving a minimum mass value from block 172. An outputof limiter 168 provides the residual mass value RM. In accordance withthe control strategy illustrated in FIG. 4, the residual mass value;i.e., the mass of residual gases trapped in the number of cylinders ofengine 12, is estimated according to the equation:

RM=(MDC*EXP)/EXT(° R)  (4),

[0064] where,

[0065] MDC is the mass density constant,

[0066] EXP is the exhaust gas pressure, and

[0067] EXT is the exhaust gas temperature.

[0068] Referring now to FIG. 5, one preferred embodiment of thein-cylinder oxygen estimator block 154 of FIG. 3, in accordance with thepresent invention, is shown. Block 154 includes a first function block180 receiving the engine speed signal from engine speed sensor 62 andcomputing an engine speed factor ESF according to a function f1. In oneembodiment, the function f1 preferably produces an engine speed factorESF according to the equation:

ESF=K1/[(NCYL/REVS _(—) CYCLE)*ES)  (5),

[0069] where,

[0070] K1 is a constant (e.g., 453257.77 mg/lbm),

[0071] NCYL is the number of cylinders in the engine 12,

[0072] REVS_CYCLE is the number of crankshaft revolutions per cycle(e.g., 2), and

[0073] ES is the engine speed (RPM).

[0074] The output of function block 180 is provided to a firstmultiplication input of an arithmetic block 182 having a secondmultiplication input receiving the charge flow value CHF from either thecharge flow estimation strategy illustrated in FIG. 2 or from mass flowsensor 70. The output of arithmetic block 182 produces a charge massvalue CM as the product of charge flow CHF and the engine speed functionESF. The output of function block 180 is also provided to a firstmultiplication input of another arithmetic block 184 having a secondmultiplication input receiving the EGR mass flow value EGRF from eitheran EGR mass flow estimation algorithm or from mass flow sensor 74. Theoutput of arithmetic block 184 produces an EGR mass value EGRM as theproduct of EGR mass flow EGRF and the engine speed function ESF.

[0075] The charge mass value CM produced by block 182 is provided to anaddition input of arithmetic block 186 and the EGR mass value EGRMproduced by block 184 is provided to a subtraction input of block 186,wherein the output value MA produced by block 186 is the mass of air inthe charge supplied to intake manifold 14. The mass air value MA isprovided as an input to a function block 188 having a function f3producing a value corresponding to the mass of oxygen in the air mass(O2MA) according to the equation:

O2MA=O2DA*MA  (6),

[0076] where,

[0077] O2DA is a conversion factor corresponding to a typical fractionof oxygen in dry air (e.g., 0.2319), and

[0078] MA is the mass of air value provided by block 186.

[0079] The commanded fueling value CF is provided to a first input of asummation block 190 having a second input receiving the charge massvalue CM, wherein the output of block 190 corresponds to the mass ofcharge plus fuel (C+FM). A MAX block 192 has a first input receiving theC+FM value from block 190 and a second input receiving a constant K1from block 194, wherein the output of block 192 corresponds to a maximumof C+FM and a minimum C+FM value K1. The output of MAX block 192 isprovided to a division input of an arithmetic block 196.

[0080] The commanded fueling value CF is also provided to an input of asecond function block 198 producing an air/fuel ratio oxygen value (AFO)according to the equation:

AFO=CF*STOICAFR*O2DA  (7),

[0081] where,

[0082] CF is commanded fueling,

[0083] STOICAFR is a constant corresponding to a stoichiometric air/fuelratio, and is a calibratable constant depending upon fuel type, and

[0084] O2DA is a conversion factor corresponding to a typical fractionof oxygen in dry air (e.g., 0.2319).

[0085] The AFO value is provided as a first input to a known limiterblock 200 having a second input receiving a minimum value MIN from block202 and a third input receiving a maximum value O2IC (corresponding tothe in-cylinder oxygen value output of block 154) from the output ofsummation block 230. The output O2MB of the limiter block 200corresponds to the mass of oxygen required to burn the fuel trapped inthe engine cylinders for the current combustion event, and is providedto a subtraction input of an arithmetic block 204 having a summationinput receiving the O2IC value from the output of block 230. The outputof block 204 is provided to a multiplication input of arithmetic block196 having an output RO2F corresponding to the fraction of oxygen in theresidual gases trapped in the cylinders of the engine 12.

[0086] A multiplication block 208 has a first input receiving the enginespeed signal ES and a second input receiving the volumetric efficiencyvalue η from the charge flow estimation strategy of FIG. 2. The outputof block 208 is provided to a first input of a MAX block 210 having asecond input receiving a constant K2 from block 212. The maximum valueof K2 and the output of block 208 is provided to a fourth function block214 producing a delay value D according to the equation:

D=(VFRAC*K)/(η*ES)  (8),

[0087] where,

[0088] VFRAC is a scaling value,

[0089] K is a conversion constant (e.g., 120 rev*sec/cycle*min),

[0090] η is the volumetric efficiency of the intake manifold 14, and

[0091] ES is the engine speed (RPM).

[0092] The output D of function block 214 is provided to a first inputof a delay block 216 having a second input receiving the residual oxygenfraction value RO2F from block 196, and providing a delayed output to aninput of a filter block 218. The filter block 218 is preferably afirst-order filter having a filter constant FC provided by block 218,although the present invention contemplates using other known filteringtechniques for filter 218. In any case, the output of filter 218(O2EGRF) represents the fraction of oxygen in the recirculated exhaustgas and is provided to a first input of a multiplication block 222having a second input receiving the EGR mass value EGRM.

[0093] Delay block 216 is operable to delay the residual oxygen fractionvalue RO2F by an amount defined by the delay value D, wherein the delayis thus a function of engine speed ES and volumetric efficiency η. Thedelay provided by blocks 208-218 is intended to account for a transportlag between the EGR valve 38 and intake manifold 14, wherein the delayis based on the time necessary to move one displacement volume of fluidthrough the engine at a given volumetric efficiency. The scaling valueVFRAC is operable to scale this delay time according to thedisplacement/EGR line volume ratio. The O2EGRF value thus incorporates adelay defined by the transport lag between the EGR valve 38 and theintake manifold 14.

[0094] The output of multiplication block 222 is provided to a firstinput of a summation block 224 having a second input receiving theoutput of a multiplication block 228. A first input of multiplicationblock 228 receives the residual mass value RM provided by block 152 ofFIGS. 3 and 4, and a second input of block 228 receives a delayedresidual oxygen fraction value (RO2F) from delay block 226. Delay block226 is preferably operable to delay the residual oxygen fraction valueRO2F and by one combustion cycle and provide this delayed RO2F value toblock 228, wherein the output of block 228 is provided to the secondinput of summation block 224. The output of summation block 224 providesa value O2ME corresponding to the total mass of oxygen in thein-cylinder exhaust gas, and this value is provided to one input ofsummation block 230 having a second input receiving the O2MA value (massof oxygen in the fresh air trapped in the engine cylinder. The output ofblock 230 defines the in-cylinder oxygen value O2IC produced by block154.

[0095] The in-cylinder oxygen mass estimation strategy illustrated inFIG. 5 is, in accordance with the present invention, based on anestimation of the mass of oxygen in the trapped (in-cylinder) charge forthe current combustion cycle plus the mass of oxygen in the trappedresidual gases resulting from the previous combustion event. Morespecifically, the total mass of oxygen trapped in the cylinders of theengine for the current (kth) combustion cycle (ICO2_(K)) is estimatedaccording to the equation:

ICO2_(K) =O2MA _(K) +O2ME _(K)  (9),

[0096] where,

[0097] O2MA is the mass of oxygen in the fresh air portion of the chargetrapped in the engine cylinders for the current (kth) combustion cycle,and

[0098] O2ME is the mass of oxygen in the exhaust gas portion of thecharge trapped in the engine cylinders for the current (kth) combustioncycle.

[0099] The first term, O2MA_(K), is preferably computed as a standardfraction of oxygen in a dry air mass, wherein the dry air mass forequation (9) is computed as a difference between the values of thecharge mass and the EGR mass trapped in the cylinders of the engine forthe current (kth) combustion cycle. O2MA is thus preferably computedaccording to the equation:

O2MA _(K) =O2DA*(CM _(K) −EGRM _(K))  (10),

[0100] where,

[0101] O2DA is a conversion factor corresponding to a typical fractionof oxygen in dry air (e.g., 0.2319),

[0102] CM_(K) is the mass of charge trapped in the cylinders of theengine for the current (kth) combustion cycle, and

[0103] EGRM_(K) is the mass of recirculated exhaust gas trapped in thecylinders of the engine for the current (kth) combustion cycle.

[0104] It will be noted that equation (10) is the output of functionblock 188 of FIG. 5, and is identical to equation (6) above with the airmass value MA of equation (6) represented in terms of CM and EGRM.

[0105] The second term, O2ME_(K), in equation (9) above is preferablyestimated according to the equation:

O2ME _(K)=(RM _(K) *RO2F _(K))+(EGRM _(K) *O2EGRF _(K-m))  (11),

[0106] where,

[0107] RM_(K) is the mass of residual gases from the previous combustioncycle now trapped in the cylinders of the engine for the current (kth)combustion cycle,

[0108] RO2F_(K) is the fraction of oxygen in the residual gases from theprevious combustion cycle now trapped in the cylinders of the engine forthe current (kth) combustion cycle,

[0109] EGRM_(K) is the mass of recirculated exhaust gas trapped in thecylinders of the engine for the current combustion cycle, and

[0110] O2EGRF_(K-m) is the fraction of oxygen in the mass ofrecirculated exhaust gas, wherein O2EGRF_(K-m) is delayed by a timeperiod of “m” engine cycles (m may be any positive integer)corresponding to the transport lag between the EGR valve 38 and intakemanifold 14.

[0111] It will be noted that O2ME_(K) is provided as the output ofsummation block 224 of FIG. 5, and that the outputs of blocks 224 and188 are combined at block 230 to define the total in-cylinder oxygenmass value O2IC.

[0112] The RO2F_(K) term of equation (11) is preferably estimatedaccording to the equation:

RO2F _(K)=(O2MA _(K-1) −O2MB _(K-1) +O2ME _(K-1))/(C+FM _(K-1))  (12),

[0113] where,

[0114] O2MA_(K-1) is the mass of oxygen in the fresh air portion of thecharge trapped in the engine cylinders in the previous (k−1)thcombustion cycle,

[0115] O2MB_(K-1) is the mass of oxygen required to burn the fueltrapped in the cylinders in the previous (k−1)th combustion cycle,

[0116] O2ME_(K-1) is the mass of oxygen in the exhaust gases trapped inthe cylinders for the previous (k−1)th combustion cycle, and

[0117] C+FM_(K-1) is the sum of charge and fuel masses for the previous(k−1)th combustion cycle.

[0118] Substituting equation (11) into equation (12) yields theequation:

RO2F _(K)=(O2MA _(K-1) −O2MB _(K-1) +RM _(K-1) *RO2F _(K-1) +EGRM _(K-1)*O2EGRF _(K-m-1))/(C+FM _(K-1))  (13).

[0119] It will be noted that RO2F_(K) from equation (13) represents theoutput of the delay block 226 of FIG. 5.

[0120] Referring now to FIG. 6, one preferred embodiment of the OFC fuellimit estimator block 156 of FIG. 3, in accordance with the presentinvention, is shown. Block 156 includes an equivalence ratio limitdetermination block 240 having a first input receiving the defaultfueling value F_(DEF) from the default fueling block 78 of FIG. 1, asecond input receiving the engine speed signal (ES) from engine speedsensor 62 and an output producing a maximum equivalence ratio value(Φ_(MAX)) as a function thereof. In one preferred embodiment, Φ_(MAX)values are predetermined as functions of engine speed (ES) and defaultfueling (F_(DEF)), and block 240 represents is a three-dimensional tableor graph defining a number of Φ_(MAX) values stored therein as afunction of ES and F_(DEF) table or graph axes. An exampletwo-dimensional slice of one such graph of Φ_(MAX) values is illustratedin FIG. 7 for two different engine types. For example, curve 260represents a plot of Φ_(MAX) values vs. engine speed (RPM) and commandedfueling (not shown) for one engine type while curve 270 represents anΦ_(MAX) map for another engine type. It is to be understood that whilethe equivalence ratio limit determination block 240 of FIG. 6 has beendescribed and represented in FIG. 7 as a table or graph, the presentinvention contemplates that block 240 may alternatively include one ormore equations relating commanded fueling (F_(DEF)) and engine speed(ES) to a maximum equivalence ratio value Φ_(MAx).

[0121] In any case, the Φ_(MAX) output of block 240 is provided to amultiplication input of an arithmetic block 242 having a secondmultiplication input receiving the in-cylinder oxygen mass value fromthe in-cylinder oxygen estimator block 154. A division input of block242 receives a stoichiometric oxygen/fuel ratio constant STOIC OF RATIOfrom block 244, wherein STOIC OF RATIO is a calibratable constantdefined by fuel type. The resulting OFC fueling command produced byarithmetic block 242 is provided as an input to a known limiter block246 receiving a minimum OFC fueling value OFC MIN from block 248 and amaximum OFC fueling value OFC MAX from block 250. Limiter block 246 isoperable to produce the OFC limit fueling command F_(OFCL) as the OFCfueling command produced by block 242 having a maximum value of OFC MAXand a minimum value of OFC MIN. The OFC fuel limit estimator is thusoperable to use the in-cylinder oxygen mass value produced by thein-cylinder oxygen estimator block 154, along with a maximum equivalenceratio value Φ_(MAX) computed as a function of current engine speed andfueling command values, to compute a fuel limit value F_(OFCL) thatlimits the equivalence ratio (ratio of fuel-to-oxygen) to Φ_(MAX) andaccordingly constrains particulate emissions below a desired emissionslevel.

[0122] Referring now to FIG. 8, a flowchart is shown illustrating aprocess 300 for carrying out the OFC fuel limiting function justdescribed with respect to FIGS. 1-7. The process begins at step 302 andat step 304, a number of maximum equivalence ratio values Φ_(MAX) arestored in block 240 as functions of engine speed (ES) and fuelingcommand values (F_(DEF)). Step 304 thus corresponds to providing for agraph, table or one or more equations relating Φ_(MAX) to engine speedand commanded fueling as described with respect to FIG. 6. Thereafter atstep 306, the engine controller 42 is operable to determine a currentvalue of engine speed (ES) preferably by monitoring the output of theengine speed sensor 62. Thereafter at step 308, the engine controller 42is operable to determine a current default fueling value (F_(DEF))preferably from the default fueling block 78 of FIG. 1.

[0123] From step 308, process 300 advances to step 310 where the enginecontroller 42 is operable to map the values of engine speed (ES) anddefault fueling (F_(DEF)) determined at steps 306 and 308 respectivelyto a maximum equivalence ratio Φ_(MAX) preferably using block 240 ofFIG. 6. Thereafter at step 312, the engine controller 42 is operable todetermine the in-cylinder oxygen mass (O2) preferably as described withrespect to FIGS. 4 and 5, and at step 314 the engine controller 42 isoperable to determine a maximum OFC fueling value (F_(OFCL)) as afunction of the Φ_(MAX) and O2 values determined at steps 310 and 312respectively, and preferably as described with respect to FIG. 6.Thereafter at step 316, the engine controller 42 is operable to limitengine fueling based on the maximum OFC fueling value F_(OFCL). In onepreferred embodiment of step 316, the engine controller 42 is operableto compare the OFC fueling value F_(OFCL) with the default fueling valueF_(DEF) and fuel the engine 12 based on a minimum of the two fuelingvalues. Those skilled in the art will recognize other known fuellimiting techniques for limiting engine fueling based on the OFC fuelingvalue F_(OFCL), and any such techniques are intended to fall within thescope of the present invention. In any case, process 300 preferablyloops back to step 306 following execution of step 316.

[0124] While the invention has been illustrated and described in detailin the foregoing drawings and description, the same is to be consideredas illustrative and not restrictive in character, it being understoodthat only preferred embodiments thereof have been shown and describedand that all changes and modifications that come within the spirit ofthe invention are desired to be protected.

What is claimed is:
 1. An equivalence ratio-based system for controllingtransient fueling in an internal combustion engine, comprising: anengine speed sensor producing an engine speed signal indicative ofrotational speed of an internal combustion engine; means for determininga quantity of oxygen trapped within a number of cylinders of said engineand producing an oxygen estimate corresponding thereto; and a controlcircuit producing a fueling command for fueling said engine anddetermining a maximum equivalence ratio value based on said fuelingcommand and said engine speed signal, said control circuit limiting saidfueling command based on said maximum equivalence ratio and said oxygenestimate.
 2. The system of claim 1 further including a fueling systemresponsive to a final fueling command to supply fuel to said engine;wherein said control circuit is operable to determine a limited fuelingsignal as a function of said maximum equivalence ratio and said oxygenestimate, and to produce said final fueling command as a minimum of saidfueling command and said limited fueling command.
 3. The system of claim2 wherein said control circuit includes a number of maximum equivalenceratio values stored therein as functions of different values of saidfueling command and of different values of said engine speed signal; andwherein said control circuit is operable to determine a maximumequivalence ratio value by mapping current values of said fuelingcommand and said engine speed signal to a stored one of said maximumequivalence ratio values.
 4. The system of claim 2 wherein said controlcircuit includes a predefined stoichiometric oxygen-fuel ratio valuestored therein; and wherein said control circuit is operable todetermine said limited fueling command as a product of said maximumequivalence ratio and said oxygen value divided by said predefinedstoichiometric oxygen-fuel ratio value.
 5. An equivalence ratio-basedsystem for controlling transient fueling in an internal combustionengine, comprising: an engine speed sensor producing an engine speedsignal indicative of rotational speed of an internal combustion engine;means for determining a residual mass value corresponding to a mass ofresidual gases trapped within a number of cylinders of said engine;means for producing a fueling command for fueling said engine; meansresponsive to said residual mass value, said engine speed signal andsaid fueling command for determining a quantity of oxygen trapped withinsaid number of cylinders of said engine and producing an oxygen valuecorresponding thereto; and a control circuit limiting said fuelingcommand based on said engine speed signal, said fueling command and saidoxygen value.
 6. The system of claim 5 further including means fordetermining a mass flow of charge entering an intake manifold of saidengine and producing a charge flow value corresponding thereto; andwherein said means for determining a quantity of oxygen is furtherresponsive to said charge flow value for determining said quantity ofoxygen trapped within said number of cylinders of said engine andproducing said oxygen value corresponding thereto.
 7. The system ofclaim 5 further including means for determining a volumetric efficiencyof an intake manifold of said engine and producing a volumetricefficiency value corresponding thereto; and wherein said means fordetermining a quantity of oxygen is further responsive to saidvolumetric efficiency value for determining said quantity of oxygentrapped within said number of cylinders of said engine and producingsaid oxygen value corresponding thereto.
 8. The system of claim 5further including means for determining a mass flow of exhaust gasrecirculated from an exhaust manifold to an intake manifold of saidengine and producing an EGR flow value corresponding thereto; andwherein said means for determining a quantity of oxygen is furtherresponsive to said EGR flow value for determining said quantity ofoxygen trapped within said number of cylinders of said engine andproducing said oxygen value corresponding thereto.
 9. The system ofclaim 5 further including: means for determining a mass flow of chargeentering an intake manifold of said engine and producing a charge flowvalue corresponding thereto; means for determining a volumetricefficiency of said intake manifold and producing a volumetric efficiencyvalue corresponding thereto; and means for determining a mass flow ofexhaust gas recirculated from an exhaust manifold of said engine to saidintake manifold and producing an EGR flow value corresponding thereto;and wherein said means for determining a quantity of oxygen includesmeans for estimating said quantity of oxygen based on said engine speedsignal, said fueling command, said charge flow value, said volumetricefficiency value and said EGR flow value, and producing said oxygenvalue corresponding thereto.
 10. The system of claim 5 further includingmeans for determining an exhaust pressure value corresponding to apressure of exhaust gases produced by said engine; wherein said meansfor determining a residual mass value includes means responsive to saidexhaust pressure value for determining said residual mass value.
 11. Thesystem of claim 10 further including means for determining an exhausttemperature value corresponding to a temperature of exhaust gasesproduced by said engine; wherein said means for determining a residualmass value further includes means responsive to said exhaust temperaturevalue for determining said residual mass value.
 12. The system of claim11 wherein said means for determining a residual mass value includesmeans for estimating said residual mass value as a product of saidexhaust pressure value and a predefined mass density constant divided bysaid exhaust temperature value.
 13. An equivalence ratio-based methodfor controlling transient fueling in an internal combustion engine,comprising the steps of: sensing rotational speed of an internalcombustion engine and producing an engine speed signal correspondingthereto; determining a maximum equivalence ratio value based on anengine fueling command and said engine speed signal; determining aquantity of oxygen trapped within a number of cylinders of said engineand producing an oxygen value corresponding thereto; and limiting fuelsupplied to said engine command based on said maximum equivalence ratioand said oxygen value.
 14. The method of claim 13 wherein the limitingstep further includes: determining a limited fueling signal as afunction of said maximum equivalence ratio and said oxygen value;producing a final fueling command as a minimum of said engine fuelingcommand and said limited fueling command; and fueling said engineaccording to said final fueling command.
 15. The method of claim 13further including the step of storing a number of maximum equivalenceratio values as functions of different values of said engine fuelingcommand and of different values of said engine speed signal; and whereinthe determining step includes determining said maximum equivalence ratiovalue by mapping current values of said engine fueling command and saidengine speed signal to a stored one of said maximum equivalence ratiovalues.
 16. The method of claim 14 further including the step of storinga predefined stoichiometric oxygen-fuel ratio value; and wherein thelimiting step includes limiting fuel supplied to said engine based on aproduct of said maximum equivalence ratio and said oxygen value dividedby said predefined stoichiometric oxygen-fuel ratio value.
 17. Anequivalence ratio-based method for controlling transient fueling in aninternal combustion engine, comprising the steps of: sensing rotationalspeed of an internal combustion engine and producing an engine speedsignal corresponding thereto; determining a residual mass valuecorresponding to a mass of residual gases trapped within a number ofcylinders of said engine; producing a fueling command for fueling saidengine; determining a quantity of oxygen trapped within said number ofcylinders of said engine based on said engine speed, said residual massvalue and said fueling command and producing an oxygen valuecorresponding thereto; and limiting said fueling command based on saidengine speed signal, said fueling command and said oxygen value.
 18. Themethod of claim 17 further including the step of determining a mass flowof charge entering an intake manifold of said engine and producing acharge flow value corresponding thereto; and wherein the step ofdetermining a quantity of oxygen includes determining said quantity ofoxygen based further on said charge flow value.
 19. The method of claim17 further including the step of determining a volumetric efficiency ofan intake manifold of said engine and producing a volumetric efficiencyvalue corresponding thereto; and wherein the step of determining aquantity of oxygen includes determining said quantity of oxygen basedfurther on said volumetric efficiency value.
 20. The method of claim 17further including the step of determining a mass flow of exhaust gasrecirculated from an exhaust manifold to an intake manifold of saidengine and producing an EGR flow value corresponding thereto; andwherein the step of determining a quantity of oxygen includesdetermining said quantity of oxygen based further on said EGR flowvalue.
 21. The method of claim 17 further including the steps of:determining a mass flow of charge entering an intake manifold of saidengine and producing a charge flow value corresponding thereto;determining a volumetric efficiency of said intake manifold andproducing a volumetric efficiency value corresponding thereto; anddetermining a mass flow of exhaust gas recirculated from an exhaustmanifold of said engine to said intake manifold and producing an EGRflow value corresponding thereto; and wherein the step of determining aquantity of oxygen includes estimating said quantity of oxygen based onsaid engine speed signal, said fueling command, said charge flow value,said volumetric efficiency value and said EGR flow value, and producingsaid oxygen value corresponding thereto.
 22. The method of claim 17further including the step of determining an exhaust pressure valuecorresponding to a pressure of exhaust gas produced by said engine; andwherein the step of determining a residual mass value includesdetermining said residual mass value based on said exhaust pressurevalue.
 23. The method of claim 22 further including the step ofdetermining an exhaust temperature value corresponding to a temperatureof said exhaust gas; and wherein the step of determining a residual massvalue includes determining said residual mass value based further onsaid exhaust temperature value.
 24. The system of claim 23 wherein thestep of determining a residual mass value includes estimating saidresidual mass value as a product of said exhaust pressure value and apredefined mass density constant divided by said exhaust temperaturevalue.